This invention relates in general to the manufacture of journal crosses for use in universal joints. In particular, this invention relates to an improved structure for a case hardened journal cross for use in a universal joint and to a method of manufacturing same.
In most land vehicles in use today, a drive train assembly is provided for transmitting rotational power from an output shaft of an engine/transmission assembly to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle. To accomplish this, a typical vehicular drive train assembly includes a hollow cylindrical driveshaft tube having first and second ends. A first universal joint is connected between the output shaft of the engine/transmission assembly and the first end of the driveshaft tube, while a second universal joint is connected between the second end of the driveshaft tube and the input shaft of the axle assembly. The universal joints provide a rotational driving connection from the output shaft of the engine/transmission assembly through the driveshaft tube to the input shaft of the axle assembly, while accommodating a limited amount of angular misalignment between the rotational axes of these three shafts.
Each of the universal joints usually includes a first yoke, a second yoke, and a journal cross connected therebetween. A typical journal cross includes a central body portion having four cylindrical trunnions extending outwardly therefrom. The trunnions are oriented in a single plane and extend at right angles relative to one another. A hollow cylindrical bearing cup is mounted on the end of each of the trunnions. Needle bearings or similar friction reducing structures are usually provided between the outer cylindrical surfaces of the trunnions and the inner cylindrical surfaces of the bearing cups to facilitate rotational movement of the bearing cups relative to the trunnions. The bearing cups that are mounted on a first opposed pair of the trunnions of the journal cross are connected to the first yoke, while the bearing cups that are mounted on a second opposed pair of the trunnions are connected to the second yoke.
The journal crosses of the universal joints subjected to two major types of forces during operation of the drive train assembly. First, when torque is transmitted through the universal joint, relatively large bending forces are applied generally throughout each of the trunnions. Such relatively large bending forces tend to flex the trunnions laterally from the right angular orientation described above. To accommodate these relatively large bending forces, it is desirable that the central body portion and the trunnions of the journal cross be formed from a material that is sufficiently strong to absorb the bending loads, yet soft enough to allow some flexing of the trunnions relative to the central body portion to avoid undesirable brittleness. Second, because the various shafts of the drive train assembly are usually axially mis-aligned during rotation as described above, the bearing cups are constantly rotated in a reciprocating manner relative to the associated trunnions. Because of the rolling engagement of the needle bearings resulting from such constant rotation, relatively small and continuous contact forces are applied locally to the outer cylindrical surfaces of the trunnions. To accommodate these relatively small contact forces, it is desirable that the outer surfaces of the trunnions be formed from a relatively hard material that is resistant to undesirable wear.
To address these competing considerations, it has been found desirable to form the journal cross of a universal joint from a material having a relatively soft interior portion (to permit desirable limited flexing of the trunnions) and a relatively hard exterior portion on the outer surfaces of the trunnions (to prevent undesirable wear from the needle bearings). Such a structure can be referred to as a case hardened journal cross, wherein the journal cross has a relatively thin outer layer (referred to as the case) that is significantly harder than the remaining inner regions thereof (referred to as the core). Thus, case hardened journal crosses have a hardened case that generally follows the contour of the central body portion and trunnions, rather than penetrating deeply therein to the core thereof.
One known method for manufacturing a case hardened journal cross is the process of carburization. To perform carburization, a journal cross is initially formed from a medium hardenability steel alloy, such as 8620 alloy steel. The journal cross is heated to a relatively high temperature in the presence of a carbon enriched atmosphere for a period of time, then subsequently cooled. The heating and cooling of the gear causes the entire journal cross (both the case and the core) to become hardened. The magnitude of the core hardening is dependent, among other things, upon the initial content of carbon in the steel, the temperature to which the journal cross is heated, and the rate of cooling. With respect to the case, however, the high temperature causes carbon from the atmosphere to be diffused into the surface of the journal cross. This carbon diffusion causes the case of the journal cross to become more hardened than the core of the gear when subsequently cooled. The depth of carbon penetration into the journal cross (and, therefore, the depth of the hardened case) is directly proportional to the magnitude of the temperature to which the journal cross is treated and the time duration of such treatment.
Although carburization has been used effectively to manufacture case hardened journal crosses for many years, it has been found to be somewhat inefficient in the context of modern production practices. Specifically, carburization is a process that requires a relatively large capital investment for performing the treatment process, including heating, handling, and cooling the parts. Also, carburization is a relatively slow process to perform, typically requiring five to ten hours to perform for each journal cross. Furthermore, carburization is a process that is best suited for simultaneous treatment of a relatively large quantity of identical parts, resulting in an undesirably large quantity of treated parts that must be stored in temporary inventory until they can be consumed. Thus, it would be desirable to provide an alternative method for manufacturing a case hardened journal cross for use in a universal joint.
This invention relates to an improved structure for a case hardened journal cross for use in a universal joint and to a method of manufacturing same. In particular, the current invention provides an apparatus and method to optimize the depth and location of the case hardened layer using an inductor that is shaped to facilitate the manufacturing process. The apparatus includes an inductor that is specifically designed with one open side to allow the journal cross to be inserted and withdrawn from the induction process with relative ease. The conductive frame of the inductor is shaped in a cross-like pattern that outlines the outer perimeter of a journal cross on each side of the journal cross. Because the induction coil passes across both sides of the journal cross, heat treating may be enhanced at specific locations of the outer is surface of the journal cross by adjusting the distance of the induction coil from the journal cross. This is particular effective at optimizing the case hardened layer at specific locations as determined by computer modeling or other techniques. The present invention discloses a method for manufacturing a journal cross for a universal joint with optimized location and depth of the case hardened layer. The method first involves the step of determining the maximum stress distribution for the journal cross, using computer modeling or other techniques, based on the intended application in which the journal cross is to be applied. Once the maximum stress distribution has been determined, the desired depth and location of the case hardening is determined. In particular, the depth of the case hardening throughout each of the trunnions is calculated based upon the stress distribution as being greater than or equal to one-fourth of the outer diameter of the trunnion less the inner diameter of the lubrication passageway formed through the trunnion. In the final step, the heat treatment is performed using the apparatus described above to achieve the desired case hardened layer throughout the trunnion for the particular application in which the universal joint is to be used.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.